Typically, aircraft will approach an airport under the guidance of air traffic controllers. The air traffic controllers are tasked with ensuring the safe arrival of aircraft at their destination, while also ensuring the capacity of the airport is maximised. The former requirement is generally met by ensuring minimum specified separations are maintained between aircraft.
Air traffic control is subject to uncertainties that may act to erode the separation between aircraft such as variable winds, both in speed and direction, and different piloting practices. Nonetheless, large numbers of aircraft can operate safely confined in a relatively small space since air traffic control can correct for these uncertainties at a tactical level using radar vectoring, velocity change and/or altitude change. As a result, a typical approach to an airport will involve a stepped approach where the aircraft is cleared to descend in steps to successively lower altitudes as other air traffic allows.
An alternative is to fly continuous descent approaches into airports, as this provides significant advantages. For example, air traffic noise around airports has important social, political and economical consequences for airport authorities, airlines and communities. Continuous descent approaches are an affordable way to tackle this noise problem as they reduce the number of aircraft that fly over sensitive areas at low altitude with high thrust settings and/or with non-clean aerodynamic configurations (e.g. with landing gear and/or flaps deployed). In contrast, conventional step-down approaches act to exacerbate this problem as aircraft are held at low altitudes, where engine thrust must be sufficient to maintain level flight. Continuous descent approaches also benefit fuel efficiency and minimise flight time.
However, continuous descent approaches create problems for air traffic control. The approaches must be planned in detail and then variations in the expected flight history (e.g. due to the prevailing wind conditions) cannot be subjected to tactical corrections to ensure safe aircraft separation like those used in conventional step-down approaches. Generally, this means air traffic controllers must impose larger separations between aircraft to guarantee that the aircraft arrive at the airport separated by a safe distance, bearing in mind the potential differences in aircraft separation as a result of wind changes and other uncertainties. Such an increase in separation results in an undesirable reduction in airport capacity.
European patent application number EP07380053.4, published as EP 1962256 A1 and commonly assigned, provides further background on continuous descent approaches, and describes a way of minimising uncertainties in the position and ground speed histories of aircraft flying continuous descent approaches such that greater certainty in arrival times is obtained. This allows air traffic controllers to safely reduce the separation between aircraft, thus satisfying the capacity needs of modern airports. This is achieved by maintaining a constant aerodynamic flight path angle while flying the continuous descent approach. This produces the lowest uncertainty in arrival time of aircraft, and offers a significant advantage over the previously favoured control law of maintaining a constant airspeed.
The aerodynamic flight path angle to be maintained may be chosen to produce the minimal variation in coefficient of lift. Put another way, an aerodynamic flight path angle may be determined that shows only minimal variation between the top and bottom of descent of the continuous descent approach. The optimum aerodynamic flight path angle is likely to vary for any particular aircraft type, and may even vary for different models within that type. Further parameters are also likely to affect the optimum aerodynamic flight path angle, such as the weight of the aircraft, the expected wind and wind gradient and the expected atmospheric conditions. The aerodynamic flight path angle to be maintained may be determined using a look-up table of data, or determined using simulations. This may be performed on the aircraft or at the airport. The airport may assign a common aerodynamic flight path angle to all aircraft types arriving at the airport, and may also assign a ground speed to be met at the top of descent.
The present disclosure aims to develop the methods described in EP07380053.4.